An exhaust gas emitted from an internal combustion engine of an automobile, etc. contains hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) or the like. Various exhaust gas-purifying catalysts for purifying these gases have been developed.
For example, Jpn. Pat. Appln. KOKAI Publication No. 01-242149 describes an exhaust gas-purifying catalyst containing an activated alumina cover layer. The activated alumina cover layer is formed using slurry containing activated alumina and at least one of nickel oxide and cobalt oxide. The activated alumina cover layer is loaded with a composite oxide of cerium oxide and zirconium oxide by impregnating the activated alumina cover layer with an aqueous solution containing a cerium salt and a zirconium salt, and heat-treating the activated alumina cover layer. The activated alumina cover layer is further loaded with one or more precious metals selected from the group consisting of platinum, palladium, and rhodium by impregnating the activated alumina cover layer with an aqueous solution containing a precious metal salt, and heat-treating the activated alumina cover layer.
Jpn. Pat. Appln. KOKAI Publication No. 10-202101 describes adding an alkaline solution and hydrogen peroxide to a solution containing cerium ion, aluminum ion, and zirconium ion to prepare a suspension in which a precursor of a composite oxide is dispersed, adding a carrier having a large specific surface area such as alumina or titania to the suspension, and firing the mixture to obtain a composite in which the composite oxide is supported by the carrier. This Literature also describes that the solution may contain one or more additives selected from alkali metal elements, alkaline-earth metal elements, and rare-earth elements and that the solution may contain one or more precious metal elements and/or one or more transition metal elements such as iron, nickel, molybdenum, and cobalt and the like.
Jpn. Pat. Appln. KOKAI Publication Nos. 2004-041866, 2004-041867, and 2004-041868 describe exhaust gas-purifying catalysts each containing a perovskite-type composite oxide. The perovskite-type composite oxide described in Jpn. Pat. Appln. KOKAI Publication No. 2004-041866 is represented by the general formula ABPdO3. The perovskite-type composite oxide described in Jpn. Pat. Appln. KOKAI
Publication No. 2004-041867 is represented by the general formula ABRhO3. The perovskite-type composite oxide described in Jpn. Pat. Appln. KOKAI Publication No. 2004-041868 is represented by the general formula A1-xA′xB1-y-zB′yPtzO3.